The environmental control system of an aircraft provides air supply, thermal control, and cabin pressurization for the crew and passengers. The atmosphere at typical jetliner cruising altitudes is generally very dry and cold, and outside air is pumped into the cabin on a long flight. Consequently, when humid air at lower altitudes is encountered and drawn in, the environmental control system dries it through the warming and cooling cycle, so that even with high external relative humidity, inside the cabin it will usually be not much higher than 10% relative humidity. Although low cabin humidity has health benefits such as preventing the growth of fungi and bacteria, the low humidity causes a drying of the skin, eyes, and mucosal membranes and contributes to dehydration, which leads to fatigue, discomfort, and health issues.